Therapeutic biologically-active nucleic acids (e.g., antisense oligonucleotides and RNA interference (RNAi) agents) can be delivered in vivo to target cells and tissues. Following delivery of such therapeutic nucleic acids, it is often difficult to determine whether and how much of the nucleic acids reached the intended target, particularly because the extent of nucleic-acid uptake by the target cells and tissues varies. As a result, determining an effective dose of a therapeutic nucleic acid can be challenging.